Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into unregulated exhaust gasses. Vehicles equipped with diesel or other lean burn engines offer the benefit of increased fuel economy. However, catalytic reduction of NOx emissions via conventional means in such systems is difficult due to the high content of oxygen in the exhaust gas.
In this regard, Selective Catalytic Reduction (SCR) catalysts, in which NOx is continuously removed through active injection of a reductant into the exhaust gas mixture entering the catalyst, are known to achieve high NOx conversion efficiency. Urea-based SCR catalysts use gaseous ammonia as the active NOx reducing agent. Typically, an aqueous solution of urea is carried on board of a vehicle, and an injection system is used to supply it into the exhaust gas stream entering the SCR catalyst. The aqueous urea decomposes to hydrocyanic acid (NHCO) and gaseous ammonia (NH3) in the exhaust gas stream. The hydrocyanic acid is catalytically converted to NH3 on the SCR. Ideally, most of the ammonia will be stored in the catalyst for reaction with the incoming NOx. NOx conversion efficiency of an SCR catalyst is improved in the presence of adsorbed ammonia. However, if the amount of ammonia stored in the catalyst is too high, some of it may desorb and slip from the catalyst. Additionally, in the presence of high temperatures, excessive ammonia storage will lead to excessive NOx via oxidation. All of this will lead to a reduction in the overall NOX conversion efficiency. Therefore, in order to achieve optimal NOx reduction and minimize ammonia slip in a urea-based SCR catalyst, it is important to control the amount of ammonia stored in the SCR catalyst.
A typical prior art system is described in U.S. Pat. No. 6,069,013, wherein a sensor is placed downstream of an SCR catalyst to detect NH3. The sensor is comprised of a low acidity zeolite material of low precious metal content. The a.c. impedance of the sensor is reduced in the presence of NH3.
The inventors herein have recognized a disadvantage with such an approach. In particular, an ammonia sensor placed downstream of the catalyst generates a signal only when there is ammonia slip over the catalyst. Ammonia slip is usually a result of temperature transients or excessive storage. Slip due to excessive storage will be impossible to rectify expediently via any control action. Hence, it is recognized that control action based solely on NH3 sensor feedback is, at best, a delayed corrective action.
Further, the inventors have recognized that the bulk of the ammonia introduced into the catalyst is stored or reduced on the upstream 20-30% of a typically sized catalyst brick on the order of 1 to 2 engine swept volumes. The remaining catalyst volume acts as a buffer to capture slip and allow some transient NOx reduction at high space velocities. Further, inventors have recognized that in order to achieve optimal NOx conversion in the SCR, it is not necessary that all of the catalyst storage capacity be utilized by ammonia. Therefore, it is desirable to either control the amount of ammonia stored in the catalyst to some optimal level below maximum (for a single brick configuration) or to store at higher levels only in the front brick/s for a multi-brick configuration.
The inventors herein have determined an improvement can be achieved by splitting the catalyst brick into at least two parts, wherein the volume of the first brick would be 20-30% of the overall single brick equivalent catalyst volume. The first brick would perform most of the ammonia storage/NOx conversion functions, and the second brick would serve to catch any of the ammonia slipping past the first brick. Thus, inventors have recognized that by controlling the amount of ammonia stored in the first brick, effective control of overall catalyst ammonia storage amounts can be achieved.
Further, inventors herein have devised a method to effectively measure and control the amount of ammonia stored in the catalyst prior to achieving catalyst saturation levels. Namely, the inventors herein have recognized that it is possible to intrusively desorb a portion of the ammonia stored in the catalyst, and to determine the overall amount of ammonia stored based on a reading of an NH3 sensor positioned in the vicinity of the desorbtion area.
Additionally, the inventors herein have recognized that it is possible to effectively diagnose system degradation in catalyst performance by monitoring and controlling the amount of ammonia stored in the catalyst. In particular, inventors have recognized that when NOx conversion efficiency of the catalyst is degraded, and the amount of ammonia storage is below optimal, injection of a predetermined amount of reductant will improve NOx conversion efficiency unless the catalyst is poisoned by hydrocarbons or thermally aged. In other words, the inventors herein have recognized that if NOx conversion efficiency of the catalyst does not improve following injection of ammonia, the catalyst performance may be degraded due to hydrocarbon poisoning and it should be regenerated.
Therefore, in accordance with the present invention, a method is presented for controlling a NOx-reducing catalyst, the method including: intrusively desorbing a portion of reductant stored in the catalyst; adjusting reductant injection into the catalyst based on an amount of reductant intrusively desorbed; and regenerating the catalyst when NOx conversion efficiency of the catalyst remains below a predetermined value for a predetermined amount of time following said reductant injection adjustment.
In yet another embodiment of the present invention, a diagnostic system, includes: an engine; a catalyst coupled downstream of said engine, including: a first catalyst brick, said brick having a heated portion; and a sensor coupled in close proximity to said heated portion; and a controller adjusting a temperature of said heated portion of said first catalyst brick to desorb reductant stored on said heated portion, said controller adjusting an amount of reductant in an exhaust gas mixture entering said catalyst based on a response of said sensor to said desorbed reductant; and providing an indication of catalyst degradation if an amount of an exhaust gas component downstream of said catalyst remains above a predetermined value for a predetermined time following said controller adjusting said amount of reductant entering said catalyst.
An advantage of the present invention is improved emission control. Another advantage of the present invention is improved vehicle diagnostic capabilities.
The above advantages and other advantages, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings and from the claims.